Hollow-fiber and spiral-wound devices employing membranes such as asymmetric polyamide, thin-film composite and cellulose acetate membranes are used in a wide variety of reverse osmosis (RO) applications. The permselective properties of these membranes permit the purification of liquid streams to eliminate undesirable dissolved components. One significant use of these membranes is desalination of seawater or brackish water.
While currently available reverse osmosis units are highly effective in their intended applications, biological activity present in liquid streams being treated can lead to biofouling, causing poor performance and limiting the life of the unit. Biofouling or clogging of the RO unit results when microorganisms coat or become embedded and multiply on the membranes.
Biological activity is dependent on site-specific factors such as temperature, pH, organic and inorganic nutrients, oxygen availability, sunlight, and pollution and/or surface water run-off. Thus, the concentration and types of microorganisms depend on the source of the water to be treated (raw water) and seasonal conditions. That is, water from seawells has significantly lower biological activity than surface seawater Also, deep surface seawater usually has less activity than shallow shoreline seawater Activity at a single site will increase during summer months.
Ridgway, et al. teach in "Biofilm Fouling of RO Membranes -Its Nature and Effect on Treatment of Water for Reuse", Journal--American Water Works Association. June 1984, pp. 94-102, the effect of chlorine disinfection of raw water from waste treatment to RO units having cellulose acetate membranes. The raw water contained a high concentration of ammonia, typically more than 15 milligrams per liter (mg/L). Effects in RO feedwater containing a low combined chlorine residual (1-3 mg/L) were compared with RO feedwater to which an additional 15-20 mg/L chlorine was added. High concentrations of combined chlorine did not prevent adhesion of bacteria to the surfaces of the membrane, even though, with one unexplained exception, no colonizing microorganisms were recovered from the membrane surfaces exposed to high combined chlorine. Dechlorination prior to passing the feed to the membrane was not employed
Applegate et al. teach in "Monitoring and Control of Biological Activity in Permasep.RTM. Seawater RO Plants", Desalination, 65 (1987), pp. 331-359, typical pretreatment to control biological activity when feeding seawater to Permasep.RTM. RO units which employ polyamide membranes. Chlorination-dechlorination coupled with sodium bisulfite shock treatment is used.
Free chlorine [hypochlorous (HOCl) and hypobromous acid (HOBr)]is taught as the main disinfecting agent. When chlorine is added to seawater, most of the HOCl reacts with the high concentration of bromide ion (Br.sup.-) in the seawater to form HOBr. For the sake of simplicity, Applegate et al. use the term "chlorine" to refer to the HOBr with some HOCl that is typically present The same terminology will be used herein in the case of seawater.
Applegate et al. teach that sufficient chlorine as a gas, a solution [NaOCl or Ca(OCl).sub.2 ] or from an electrolytic chlorine generator must be added to exceed the chlorine demand and, if present, the chlorine breakpoint. The chlorine demand is that needed to quickly chlorinate species such as Fe.sup.++, H.sub.2 S, NO.sub.2.sup.-, and certain organic compounds. The chlorine breakpoint is the point at which any chloramines (referred to as "combined chlorine") formed due to the presence of ammonia, for example, breakdown and free chlorine residuals begin to form. Dechlorination is taught as necessary since the asymmetric polyamide RO membranes employed are adversely affected (increased salt passage and decreased product flow) by chlorine Sodium metabisulfite (Na.sub.2 S.sub.2 O.sub.5) is the dechlorination agent of choice. Na.sub.2 S.sub.2 O.sub.5 shock treatments (higher concentrations for longer periods) in conjunction with chlorination-dechlorination are also taught.
Pohland et al. teach in "Successful Operation of a Permasep.RTM. Permeator Reverse-Osmosis System on Biologically Active Feed Water", American Chemical Society Symposium Series 153, Synthetic Membranes,
(1981), pp. 399-406, a method for resolving a biological fouling problem in polyaramide-membrane-equipped RO units by intermittently injecting an excess of a 10 percent solution of potassium iodide (as source of iodine) into the chlorinated feedwater while at the same time discontinuing the flow of sodium metabisulfite.
U.S Pat. No. 4,278,548 teaches a process for inhibiting the growth of microorganisms on polyamide membranes while not seriously damaging the polymeric structure of the membrane. The process introduces an additive into the process stream prior to its entering a RO module containing a semipermeable polyamide membrane for a period of 30 minutes to 3 hours per 72 hours of operation. The additive is selected from the group consisting of iodine, hydrogen peroxide, sodium persulfate, ammonium persulfate and sodium perborate.
Severe microbial fouling of RO membranes in some applications can readily occur even when chlorine is used as a biocide. This is particularly true when dechlorination is necessary such as when polyamide membranes are employed While not wishing to unnecessarily limit the invention, it has now been discovered that, in the case of seawater, this is because the chlorine oxidizes and sufficiently degrades certain organics such as the humic acid materials present in seawater into short-chain organic compounds that surviving microorganisms can use as an energy/carbon food source to rapidly grow and reproduce. With dechlorination, there is no chlorine present to check the growth of the surviving microorganisms. In non-seawater applications containing refractile compounds similar to humic acid, it is likely that the chlorine treatment would also break the compounds into assimilable energy/carbon food sources.
Humic acid material is produced by decay of algae and is generally present in higher concentrations the closer one is to shore. Humic acids are dissolved organics of a polymeric nature. They are high molecular weight compounds containing benzene and aromatic rings. Because of their refractile nature, humic acids are usually not considered to be energy/carbon sources for bacteria. Oxidizing agents such as chlorine have been found to break these compounds into lower molecular weight units which are assimilable organics useful as an energy/carbon source. Thus, the very pretreatment suggested in the art encourages rapid growth and subsequent fouling of equipment, particularly when dechlorination is required such as when the membrane would degrade in the continual presence of chlorine